The present invention relates to devices for gripping and withholding workpieces made of ferromagnetic material, for example sheet-steel elements, by means of at least one permanent magnet, said device comprising:                a supporting casing;        a magnet-holder member, carrying a main permanent magnet and mounted so that it can slide in said supporting casing between an advanced position, corresponding to an operative position of said main magnet, and a retracted position, corresponding to an inoperative position of said main magnet, and        actuator means carried at least in part by said casing, for governing a displacement of said magnet-holder member between said advanced position and said retracted position.        
A device of the type described above is for example illustrated in the document No. DE 10 2008 012 688 B3.
Devices of this type are used in industrial plants, as an alternative to traditional clamping grippers, for clamping metal workpieces or elements made of sheet metal during a machining operation, for example during execution of welding operations.
In the aforesaid known solution, the actuator means that govern the displacement of the magnet-holder member are constituted by a pneumatic cylinder.